A piezoelectric micropump is used as a cooing-water conveying pump for compact electronic devices, such as notebook computers, and also as a fuel conveying pump for fuel cells. On the other hand, a piezoelectric micro-blower is used as an air blower serving as an alternative to a cooling fan for a CPU etc., and is also used as an air blower for supplying oxygen necessary for generating electricity in fuel cells. Both the piezoelectric micropump and the piezoelectric micro-blower include a diaphragm that bends when a voltage is applied to a piezoelectric element, and have advantages of simple structure, thin profile, and low power consumption.
Typically, for conveying non-compressive fluid such as liquid, check valves made of soft material such as rubber or resin are provided at both an inlet and an outlet, and a piezoelectric element is driven at a low frequency of several tens of Hz. However, when a micropump with such check valves is used for conveying compressive fluid, such as air, the amount of displacement of the piezoelectric element is very small and fluid can be hardly discharged. Although the maximum displacement can be obtained when the piezoelectric element is driven at a frequency near a resonance frequency (first-order resonance frequency or third-order resonance frequency) of the diaphragm, since the resonance frequency is a high frequency of the order of kHz, the check valves cannot follow the displacement of the piezoelectric element. Therefore, for conveying compressive fluid, it is desirable to use a piezoelectric micro-blower having no check valve.
Patent Document 1 discloses a cooling device in which a pump chamber is formed between a pump body and a piezoelectric element, an inflow port is provided in a side surface of the pump chamber, and a discharge port is provided in a surface of the pump chamber, the surface facing the piezoelectric element. The inflow port is gradually tapered inward toward the pump chamber, while the discharge port is gradually tapered outward from the pump chamber. Since the inflow port and the discharge port are tapered as described above, the resistance of fluid passing through the inflow port is different from that of fluid passing through the discharge port. Thus, when the piezoelectric element is displaced in a direction that increases the volume of the pump chamber, fluid (e.g., air) is flown into the pump chamber through the inflow port; while when the piezoelectric element is displaced in a direction that reduces the volume of the pump chamber, fluid is discharged from the pump chamber through the outflow port. Therefore, it is possible to omit check valves for both the inflow port and the discharge port.
However, even if the inflow port and the discharge port are tapered as described above, when the piezoelectric element is displaced in the direction that increases the volume of the pump chamber, fluid is flown into the pump chamber not only through the inflow port, but also through the outflow port. Conversely, when the piezoelectric element is displaced in the direction that reduces the volume of the pump chamber, fluid is discharged not only through the outflow port, but also through the inflow port. Therefore, the total flow rate of discharge from the pump through the outflow port is smaller than the amount of change in volume of the pump chamber caused by the displacement of the piezoelectric element. Since the amount of change in volume of the pump chamber caused by the displacement of the piezoelectric element is very small, the flow rate is accordingly very low. Therefore, it is difficult for the cooling device to achieve a sufficient cooling effect.
Patent Document 2 discloses a gas flow generator that includes an ultrasonic driver having a piezoelectric disk mounted on a stainless steel disk, a first stainless steel membrane on which the ultrasonic driver is mounted, and a second stainless steel membrane mounted substantially parallel with the ultrasonic driver and spaced a predetermined distance therefrom. By applying a voltage to the piezoelectric disk, the ultrasonic driver is bent, so that air is discharged through perforations formed at the center of the second stainless steel membrane. Since the gas flow generator also has no check valve, the ultrasonic driver can be driven at high frequencies.
When the ultrasonic driver is driven at a high frequency, the gas flow generator can discharge air in a direction perpendicular to the perforations formed at the center of the second stainless steel membrane while drawing or pulling in air around the perforations, and thus can generate an inertia jet. However, the flow rate varies considerably depending on the conditions around the center perforations of the second stainless steel membrane. For example, if there is an obstacle near the center perforations, the discharge flow rate is considerably reduced. Also, if this gas flow generator is used as a cooling fan for cooling a heat source, such as a CPU, hot air around the heat source is simply blown to the heat source. This merely allows stirring of surrounding air, and thus the heat conversion efficiency is low.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2004-146547    Patent Document 2: Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2006-522896